Optical coherence tomography (abbreviated OCT) is a technique for forming a tomographic image of an object by detecting the interference light of a signal light passing through the object and a reference light. The OCT is used in, for example, the medical field due to the advantage of high resolution images being obtained quickly and non-invasively.
The major advance in this technique is Fourier domain OCT (abbreviated FD-OCT). With FD-OCT, a measurement speed several dozen to several hundred times faster compared to conventional time domain OCT (abbreviated TD-OCT) can be achieved.
FD-OCT includes spectral domain OCT (abbreviated SD-OCT) in which the interference light is detected through spectral decomposition and swept source OCT (abbreviated SS-OCT) in which interference lights of various wavelengths are obtained using a wavelength-swept light source.
The detected spectrum in SD-OCT and SS-OCT, i.e., a spectral interferogram (interference spectrum), is expressed by the following equation:
                              I          ⁢          ⁢                      (            k            )                          =                                            s              ⁡                              (                k                )                                      ·                          (                                                I                  R                                +                                  I                  S                                +                                                      ∫                                          -                      ∞                                                              +                      ∞                                                        ⁢                                      2                    ⁢                                                                                            I                          R                                                ⁢                                                  I                          S                                                                                      ⁢                                                                                  ⁢                                          cos                      ⁡                                              (                                                  kz                          +                                                                                    ϕ                              0                                                        ⁡                                                          (                              z                              )                                                                                                      )                                                              ⁢                                                                                  ⁢                                          ⅆ                      z                                                                                  )                                ⁢                                    (        1        )            
Here, k, s(k), z, IR, IS and φ0(z) represent the wave number, the light source spectrum, the path length difference between the signal arm and the reference arm, the back reflection of the reference light from the reference mirror (reference light), the autocorrelation term of the signal light passing through the object, and an initial phase term, respectively. In general, IR and IS are low frequency signals or background components (DC components) that can be easily removed. Consequently, Equation (1) is simplified as the following.
                              I          ⁡                      (                          k              ,                              n                ⁢                                      )                          =                              ∫                          -              ∞                                      +              ∞                                ⁢                                    A              ⁡                              (                z                )                                      ⁢                          cos              ⁡                              (                                  kz                  +                                                            ϕ                      0                                        ⁡                                          (                      z                      )                                                                      )                                      ⁢                                                  ⁢                          ⅆ              z                        ⁢                                              (        2        )            
Here, A(z)=s(k)·2√(IRIS) represents the back-scattering coefficient of the object at depth z. Based on Equation (2), using a Fourier-transform of I(k), the back-scattering profile of the object at the depth z, i.e., the A-line profile, can be reconstructed.
However, because the spectral interferogram is detected in real values, the reconstructed A-line profile suffers from complex conjugate ambiguity. The complex conjugate ambiguity means that a signal at z=Δz and a signal at z=−Δz cannot be differentiated from each other. As a result, image quality deteriorates as explained below.
This deterioration of image quality due to the complex conjugate ambiguity will be explained with reference to FIG. 14A-FIG. 14C. FIG. 14A shows a true image T of the object drawn in the full frame F0. However, when reconstructing the image from the interferogram consisting of a real value, the mirror image T′ (complex conjugate artifact) as well as the true image T appears as shown in FIG. 14B. In order to avoid such a complex conjugate ambiguity, the measurement depth of the object is shifted to separate the true image T and the mirror image T′ from each other (see FIG. 14C). Furthermore, only the true image T is used as a display image. That is, the mirror image T′ is discarded. Therefore, only a half of the energy (i.e. signal intensity) of the spectral interferogram contributes to the formation of the display image, and hence the brightness and contrast of the display image are reduced, and its image quality decreases. As a result, only the lower half F of the frame F0 of the reconstructed image is the display range of the true image T. That is, the upper half of the frame F0 is wasted, and the imaging depth is halved. Consequently, the demand to obtain the broadest possible image range is high.
Various techniques have been developed to remove or suppress the complex conjugate ambiguity that brings about such an issue. These techniques include phase shifting (non-patent documents 1-3), BM mode scanning (non-patent documents 4-9), frequency shifting (non-patent documents 10, 11), 3×3 fiber-optical coupler (non-patent documents 12-14), phase modulation (patent document 1, non-patent documents 15-18), etc. However, the implementation of these techniques is limited by certain practical issues as described below.
Phase shifting is a method well known in the field of Fourier optics, in which each spectral interferogram is obtained with different initial phases while moving the reference mirror stepwise by a distance on the order of light wavelengths. Phase shifting-based techniques require accurate phase changes between adjacent A-lines. These techniques are limited by devices such as a piezo stage or electrical phase modulator. Moreover, these techniques are compromised by factors such as mechanical instability of a system and chromatic errors.
BM mode scanning is a technique that is an extension of phase shifting, in which phases are changed during transverse scanning. The method of changing a phase includes a method in which the phase is changed stepwise (non-patent documents 8, 9) and a method in which the phase is changed linearly (non-patent documents 4-7). The former has some of the drawbacks of phase shifting and is not cost-effective in achieving a stepwise phase change. The latter causes an undesirable situation in which the path length is changed when the range of transverse scanning is widened.
Frequency shifting is a method based on frequency separation and is only applicable to SS-OCT. Moreover, frequency shifting requires expensive devices such as EOM (electro-optic modulators) and AOM (acousto-optic modulators) in order to shift the signal to a higher frequency band. As a result, this system also requires significantly high speed data acquisition devices.
3×3 fiber-optical coupler-based techniques suffer from wavelength-dependent coupling coefficients for broadband and require additional costly detectors.
Phase modulation techniques are more recently reported methods, in which complex conjugate artifacts are removed by adding a sinusoidal phase modulation. Phase modulation is usually introduced by a dithering mirror driven by a piezo stage provided with the reference arm. One approach relies on the integration effect of the camera, so its application is restricted to SD-OCT (non-patent document 17). Other approaches extract complex signals from multiple harmonic signals generated by the modulation based on complicated Bessel functions. This causes several major problems. For example, it requires extra decoding hardware, or higher load computation if decoding is performed by software. Moreover, at least 3 harmonic signals of different orders (from 0 order to 2nd order) are involved (non-patent documents 15, 19), and in many cases, 3rd order calibration is even necessary (non-patent documents 16, 18), but these multiple harmonic signals could easily cause aliasing. In other words, when demodulating the modulation signal in phase modulation, at least 3 signals have to be detected, so there is a problem in terms of hardware or software as well as a problem in that a higher measurement band is required in order to detect higher order modulation signals without aliasing.